During the first year of this project it was found that the acidic phospholipase A2 from H. haemachates venom is much weaker than the basic phospholipase A2 from N. nigricollis in causing convulsions and death in mice and rats. This may be due to differing penetrability and biochemical substrate specificities of the two enzymes. During the next year we propose to study the effects which specific and selective amino acid modifications have on the biochemical and pharmacological properties of these two phospholipase enzymes. The contributions of specific histidine, tyrosine, methionine, arginine and tryptophan residues to the enzymatic and pharmacological properties of these two enzymes will be studied. Pharmacological properties to be tested will include intravenous LD50 values in mice, intraventricular LD50 values in rats and effects on electrical activity of the isolated single electroplax of the electric eel. Biochemical properties to be studied will include the relative abilities of the modified enzymes to hydrolyze phospholipid substrates (singly or in mixtures) in mixed micelles lipoproteins and in red cell, electroplax and brain membranes. These studies should allow us to evaluate whether the same active site is responsible for both the enzymatic and pharmacological properties of these phospholipase preparations.